The purpose of this proposal is to examine the role of internal Ca2 + stores in neurohypophyseal nerve terminals of magnocellular hypothalamic neurons found in the posterior pituitary. These terminalssecrete Oxytocin or Arginine Vasopressin, which have well documented physiological actions on waterbalance, blood pressure, parturition and perhaps even mood. In addition single terminals can be isolatedwithout any associated structures and used for electrophysiological and optical studies of regulated exoctyosis at the organelle and molecular level. Thus, the present proposal has relevance for both clinical application and basic scientific understanding. In the classic model of exocytosis, Ca2 + enters the nerve terminal cytosol from the exterior through voltage-activated Ca2 + channels and triggers exocytosis. But, as we have demonstrated in neurohypophyseal terminals, Ca2 + may also enter the cytosol from intraterminal membrane bound Ca2 + stores. The release from stores is quantal, taking the form of Ca2 + syntillas, similar to Ca2 + sparks found in muscle. (Scintilla, Latin for spark, arising from a nerve terminal, normally a synaptic structure.) Ca2 + syntillas arise due to Ca2 + entering the cytosol through a species of internal Ca2 + channel called a ryanodine receptor (RyR), and they may occur spontaneously or be elicited. They do not trigger exocytosis. The major aim is to establish the effect of Ca2 + syntillas and Ca2 + stores on both spontaneous and elicited exocytosis. We know that syntillas do not elicit exocytosis, and our prelimary data indicates that syntillas actually inhibit exocytosis. We wish to establish this with certainty and to determine how syntillas do this. There are two general possibilities - that syntillas suppress full fusion events, allowing only partial emptying of a synaptic vesicle. Or the syntillas may suppress release of a population of larger vesicles - or both.